A private branch exchange telephone system ("PBX") provides certain advantages, such as reduced cost compared to an equal number of individual private lines. This has caused rapid growth in the number of PBX telephone systems. Increased use has spurred improvements so that PBX's are very versatile, employing state of the art digital switching techniques. Even though digital or electronic switching telephone central offices have been available for a number of years, not all central offices are electronic switching central offices. Because central offices have historically employed analog telephone circuitry, some central offices still expect to receive an analog signal. Thus, the signal emanating from a telephone user's premises must be a signal compatible with the telephone central office equipment.
A PBX can facilitate communication between various telephones on a user's premises. A trunk line connects the PBX to the central office and enables a telephone user to call a telephone which is not connected to his PBX system but is connected to the central office or to a remote PBX. The trunk line connects the PBX to the central office enabling a call to be made through the PBX over the trunk line to the central office and from the central office to an individual telephone line or to another PBX connected to the central office. The PBX can also connect telephones to another PBX on the user's premises over a tie line connecting the two PBX's. It will be appreciated that for large systems, a large number of trunk lines is required to connect a PBX to the central office or to another PBX. One trunk or one tie line is needed for each conversation; however, it is a recognized telephone industry practice to take 24 conversations and put them on just two lines, one outgoing and one incoming. When this line is the trunk between the central office and the PBX, it is referred to as a T1 link.
Telephones throughout the world have a basic digitizing rate of 8 kHz. Data on the T1 link runs at 1.544 MHz which accounts for 24 8-bit telephone conversations with 1-bit overhead. This is the T1 rate. It will be appreciated that the telephone central office anticipates data over the T1 link at the T1 data rate so that any PBX connected to the central office must provide data at the T1 data rate. Because the telephone central office is a digital system with its own clock and the PBX is a different digital system with its own clock, there will be differences in the basic timing rate even when they are supposed to be the same. Accordingly, it will be appreciated that it would be highly desirable to match the PBX data rate to the T1 data rate.
As mentioned, T1 is a 24 channel network. Modern electronic telephones are 32 channel devices that interface with T1 with 8 channels leftover. Because both T1 and the electronic telephone are working at the basic telephone system 8 kHz rate, the electronic telephone or PBX has to run at 2.048 MHz in order to get 32 telephone conversations on one line to accommodate 8-bits per telephone conversation and 32 telephone conversations. On the other hand, T1 only has to run at 1.544 MHz with 8-bits per telephone conversation because it has to accommodate only 24 conversations. Because the PBX and T1 operate at different data rates, it will be appreciated that it would be highly desirable to transfer data between T1 and the PBX while automatically compensating for the difference in the data rates.